How To Manufacture Edible Films

Edible films are used to coat fruits, vegetables and foodstuffs as a way to extend their shelf-life and protect them from injury.

The manufacturing process has developed to the extent that modern processes have made production of coatings much cheaper and within range of even the smallest development firm.

History Of Edible Films

Edible films have been with us ever since we perceived foodstuffs. All fruit for example has a peel which protects the soft internal tissues from weathering, reduces respiration, water loss, microbial and insect attack. Artificial coatings such as sausage casings, seaweed nori for wrapping sushi and rice paper for wrapping candies and sweets are much more modern contrivances. To help us further, new carbohydrates and polysaccharides, proteins and lipids have come to the market to make new coatings with wide applications.

Incidentally, all ingredients for any film must meet European Commission guidelines on regulatory and safety considerations. In the USA, Generally Recognized As Safe (GRAS) status must be granted for all ingredients if they are to be eaten.

Ingredients for edible films are sourced from the following:-

Carbohydrates Used In Edible Films

• hydroxypropylmethylcellulose (HPMC) and one of the earliest film-forming ingredients to be developed.
• starch (modified or otherwise)
• carrageenan
• carboxymethylcellulose (CMC)
• gum acacia (gum Arabic)
• pectin
• chitosan
• hydroxypropylcellulose
• pullulan,
• sodium alginate
• xanthan gum

Proteins

• casein,
• whey protein,
• soy protein,
• corn zein,
• gelatin.

These ingredients can be used alone or in combination to form the structural backbone of any film. Most films provide an oxygen barrier preventing ingress and likewise gas escape.

Lipids are sometimes added to improve the water barrier properties of the films. Lipids include:-

• waxes,
• shellac,
• fatty acids,
• fatty alcohols,
• acetylated monoglycerides.

Fibers (fibres) improve film strength, create and enhance a matte or glossy finish, and generate novel nutritional benefits. Examples of fibers include inulin, insoluble starch and pectin, psyllium, and bran and carrot fiber.

In addition, food grade plasticizers such as glycerol, sorbitol, or polyethylene glycol are added to improve film flexibility. Fruit and vegetable purees are also good film formers. A host of other ingredients mainly active additives, such as nutrients, colours, sweeteners, flavours, and breath-freshening compounds, are also often incorporated depending on the end-use application of the edible films.

The Casting Process

The first step in manufacturing an edible film is developing a film-forming formulation. Film-forming solutions are complex mixtures and commonly contain water-soluble polymers, plasticizers, fibers, colours, flavours, and sweeteners.

Low-speed to high-speed mixing and stirring are used to solubilize these ingredients depending on their nature because it must cover a wide range of viscosities and shear. In some cases heating the solution, ultrasonication and part-mixing are needed to achieve the most appropriate solutions for the casting process.

In other cases, emulsification of lipids is desired. Once the solution or emulsion is formed, a vacuum is commonly applied to remove air bubbles that may limit the strength and barrier properties of the final films. Sometimes, foamed films are required by bubbling. Bubble formation is enhanced through processing and through the addition of surfactants such as lecithin. Most formulation solution solids contents range from 20% to 35% with viscosities between 1,000 centipoise and 10,000 centipoise.

Film-forming solutions are then poured onto plates made from glass, Teflon, polyethylene, and other materials. The film is drawn down over the plate to a uniform thickness using draw-down blades or rods. Coated plates are then placed into convection ovens for rapid drying at riased temperatures. In some cases, multilayer films are created by sequentially applying and drying solutions onto films.

Once an acceptable formulation and process conditions are identified in the laboratory, the process is scaled up either on a pilot line or in a commercial manufacturing facility. Converting the aqueous solution into an edible film is essentially a three-step process involving coating, followed by drying, and finally converting.

Knife-Over-Roll Coating

Knife-over-roll coating is commonly used to commercially manufacture edible films. It is usually employed when a substantial and level coating of medium to high consistency is required. Ideal for uneven surfaces.  In this method, the stationary, rigid knife applies a precise layer of solution onto a casting substrate moving under the knife. The wet film thickness is controlled through micrometer adjustments in the height of the knife above the casting substrate. Excess solution is held in a pool behind the knife. Since a roll or drum is normally positioned below the knife, the process is known as knife-over-roll coating. Important factors to consider in optimizing the coating step include formulation, wet film thickness, viscosity, solids, temperature, number of layers, and substrate. Slot die extrusion is an alternative type of coating that can be used to coat aqueous coatings onto moving substrates.

The two types of casting lines used to manufacture edible films are either on steel belts or onto webs or flexible substrates, such as plastic films. When casting onto steel belts, the belt rotates around large drums situated at either end of the line. Solutions are applied uniformly at one end of the line and carried through the drying chamber to remove water. Stainless steel belts provide optimum uniformity, heat transfer, and drying efficiency while eliminating the expense of the flexible substrate; however, many film-forming solutions strongly adhere to the steel belt. In these cases, release substances can be added or flexible substrates can be employed on top of the steel belts. Web casting, casting directly onto flexible substrates such as polyester films or coated release papers, is the second type of casting line. Support rolls provide the web stability as it moves through the drier. Both types of casting lines allow for solutions to be dusted with powders to prevent sticking, coated with secondary thin coatings, printed with edible inks, and/or receive other treatments before being stripped from the substrate and wound into large master rolls.

The Drying Step

Drying is an essential step in forming edible films. The primary solvent being removed during drying is water; however, small amounts of volatile flavours and other ingredients may be removed as well. There are three stages during film drying (transition, constant rate, and falling rate). Zone drying is best suited to optimization of drying during each of these stages and production of high quality films free from process defects. Various types of heating can be used in the different zones, including hot air convection, hot air impingement, steam, and infrared drying. Infrared drying is particularly effective at driving off initial moisture in the films during the transition stage. Other types of heating are commonly used for the later stages of drying in zone units.

The Converting Step

The final step in the processing of edible films is converting. The large rolls of films must be converted into their final forms, such as smaller rolls, sheets, strips, or die cuts. A wide variety of slicing and slitting machines, as well as sheet and die cutters are needed to create either tailored sheets or intricate patterns to wrap the product.

The Edible Strips Market

The largest market for edible films is the film strip market. This segment was dominated by Listerine breath strips, which were introduced in 2000 and quickly hit a retail market peak at about $400 million. The breath strip market has settled down to a sustained business at about $100 million in annual sales. An additional $10 million market exists for film strips used to deliver energy, sleep aids, sore throat treatments, and other nutraceutical and dietary ingredients.

Another market for edible films is their use as decorative shapes that provide colour and flavour in foods. These can also be used to deliver nutrients and other active ingredients. This market segment is estimated at $8–$10 million in sales. The largest segment is in small menthol-flavoured squares used in toothpaste applications. Other shapes are used to decorate cakes, cookies, cereals, yogurt, and other foods.

Edible packaging films have been used to form pouches that provide premeasured amounts of ingredients, for example a vitamin and mineral enrichment for batches of bread dough. Edible pouches are also used for foods like oatmeal or drink powders. The value of this segment is estimated at $1–$2 million. Commercial edible films manufacturers for the above three markets include Innoteq, Watson, Biofilm Limited, ODF Pharma, Proinec, MonoSol, Umang Pharmatech, and others.

A recent market entry into edible films is their use as alternatives to nori, the seaweed wrap around sushi and low-calorie, gluten-free, alternatives to breads and tortillas. Flavoured fruit films are also used as glaze sheets for hams and other meat products, eliminating the need for messy liquid glazes.

An additional market for edible films is the cake decoration market. Edible inks are used to print custom designs on the edible films to produce a wide array of decorative edible sheets for baked goods. Edible films also make good plates for photographs. There are already numerous examples where whole cakes are covered in edible icing photographs.

The Future For Edible Films

In the future, new technologies like thermal extrusion and 3-D printing are being used to manufacture edible films. It is anticipated that delivery of functional bioactives, such as probiotics, nanomaterials and natural antimicrobials will be a continued growth area for edible film. Future applications of nanoscience including nanoactives and even nanocomposite films as edible films offer additional opportunities to transform their properties. It is more than likely that edible films will become the norm for synthetic and artificially constructed packaging.

General References

Han, J.H. (2014) Chapt. 9 – Edible Films and Coatings: A Review. In: Innovations in Food Packaging (2nd edt.) Academic press. pp. 213-255 (Article)